The invention relates to a piezoelectric actuators and their application within fuel injector valves. Such a piezoelectric actuator may be to actuate a control valve such as a servovalve within fuel injection valve for controlling flow of a medium. These kind of valves can be used for the direct control of an injection needle within an fuel injection valve or indirect control of a needle by means of a control valve. These kind of valves usually comprise a housing having an inlet duct and an outlet duct and, a movable body which can reciprocate in order to control communication between the inlet and the outlet ducts, and drive means for the reciprocating movement of the body.
A known valve of this kind includes a piezoelectric actuating member supported in a valve housing, the housing having an inlet duct and an outlet duct and a movable body which can be displaced by the actuating member in order to control in operation a flow of pressurized fluid through a connection between the inlet and outlet ducts, the surfaces of the movable body on which pressurized fluid can produced hydrostatic force being arranged to minimize any resulting hydrostatic force in a direction tending to oppose movement of the actuating member.
The possible extension of the piezoelectric element of such a valve and thus the potential stroke of the valve is usually small, for example, of the order to 10 or 20 micrometers. It is therefore essential to minimize the effect of thermal expansion on the valve parts so that the flow through the valve will vary only slightly with changes of ambient temperature. One solution to the problem is to choose a material for the valve housing which material has a similar thermal expansion coefficient to that of the piezoelectric element. An example of such a material is a nickel-iron alloy having a low coefficient of thermal expansion. A possible disadvantage to the use of this material is the high cost of the alloy.
U.S. Pat. No. 4,284,263 discloses a control valve having an actuating member supported within and between a closed end and a seat end of a housing, the actuating member has a fixed end connected to the close housing end and a free end connected to a movable valve body. The actuating member includes an elongated piezoelectric element which has a relatively low thermal coefficient of expansion. The housing is made from common, inexpensive materials having moderately higher thermal coefficient of expansion, and the actuating member includes a spacer having a high thermal coefficient of expansion, arranged between the closed housing end and the piezoelectric element. By proper dimensioning of the spacer and housing with respect to the piezoelectric element, temperature compensation may be achieved without the use of exotic materials. However, as shown in FIG. 2 of U.S. Pat. No. 4,284,263, the compensating structure elongates the device significantly.